The present invention relates to the field of optical associative memories.
Associative memories are attractive in that they use part of the input datum to recall the entire datum by autoassociation or another desired datum by hetroassociation. There are two main approaches to implementing all optical associative memories. The first approach relies on optical vector matrix multiplication based on spatial light modulators and Light Emitting Diodes. The second approach which operates directly on two-dimensional input images utilizes holograms as the storage media and some kind of saturable gain element or aperture as the threshold mechanism. See U.S. Pat. No. 4,860,253 issued to Owechko and Soffer. This patent has a well written discussion of prior art optical associative memory systems. Holograms are used as memory elements because of their ability to store phase and amplitude information.
Binarizing the phase allows for simple construction of a static associative memory element utilizing current VLSI techniques, or simply through utilizing spatial light modulators. Binary Phase only Filters (BPOFs) have the advantage of high light throughput while only requiring approximately 1/100 of the memory storage space of a corresponding matched holographic filter. Most holographic associative memories share a similar architecture which comprises a correlator with a thresholding device such as a retroreflecting phase conjugate mirror utilizing a photorefractive crystal such as barium titanate, apinhole array in the correlation plane with an ordinary mirror, or an optical fiber used as a spatial filter.